diff --git a/Parsedown.php b/Parsedown.php
index 8355d53..7af08c9 100755
--- a/Parsedown.php
+++ b/Parsedown.php
@@ -1008,7 +1008,7 @@ class Parsedown
 
                 case 'http':
 
-                    if (preg_match('/^https?:[\/]{2}[^\s]+\b/i', $text, $matches))
+                    if (preg_match('/^https?:[\/]{2}[^\s]+\b/ui', $text, $matches))
                     {
                         $element_url = $matches[0];
                         $element_url = str_replace('&', '&', $element_url);
@@ -1069,22 +1069,22 @@ class Parsedown
 
     private static $strong_regex = array(
         '*' => '/^[*]{2}([^*]+?)[*]{2}(?![*])/s',
-        '_' => '/^__([^_]+?)__(?!_)/s',
+        '_' => '/^__([^_]+?)__(?!_)/us',
     );
 
     private static $em_regex = array(
         '*' => '/^[*]([^*]+?)[*](?![*])/s',
-        '_' => '/^_([^_]+?)[_](?![_])\b/s',
+        '_' => '/^_([^_]+?)[_](?![_])\b/us',
     );
 
     private static $strong_em_regex = array(
         '*' => '/^[*]{2}(.*?)[*](.+?)[*](.*?)[*]{2}/s',
-        '_' => '/^__(.*?)_(.+?)_(.*?)__/s',
+        '_' => '/^__(.*?)_(.+?)_(.*?)__/us',
     );
 
     private static $em_strong_regex = array(
         '*' => '/^[*](.*?)[*]{2}(.+?)[*]{2}(.*?)[*]/s',
-        '_' => '/^_(.*?)__(.+?)__(.*?)_/s',
+        '_' => '/^_(.*?)__(.+?)__(.*?)_/us',
     );
 
     private static $special_characters = array(
@@ -1102,4 +1102,4 @@ class Parsedown
                           'span',
                           'time',
     );
-}
\ No newline at end of file
+}